1. Field of the Invention
The present invention relates to oxy-fuel combustion control, and more particularly to systems and methods for controlling oxy-fuel combustion in a furnace.
2. Brief Description of the Related Art
Oscillating Combustion (OC) to reduce NOx emissions has in the past been proposed. See Shamp, D., et al., xe2x80x9cImproving Oxy-fuel Furnace Operating Efficiency: An Operator""s Perspectivexe2x80x9d, presented at the 59th Conference on Glass Problems, Ohio State University, Columbus, Ohio, Oct. 27-28, 1998; Wagner, J., et al., xe2x80x9cOscillating Combustion Increases Productivity and Decreases NOx emissions from industrial furnacesxe2x80x9d, 1998 American-Japanese Flame Research Committees International Symposium, Maui, Hi., Oct. 11-15, 1998; Charon, O., et al., xe2x80x9cPulsated O2/Fuel flame as a new technique for low NOx emissionsxe2x80x9d, Combustion Science and Tech., Vol. 90, pp.1-1 (1993); and U.S. Pat. Nos. 4,846,665 and 5,302,111.
The oscillating combustion theory (see, e.g., FIGS. 1 and 2 of U.S. Pat. No. 4,846,665) involves the creation of successive, NOx, retarding, fuel-rich and fuel-lean zones within and along the length of the flame. This technique involves forced oscillation of the fuel flow rate to the burner. Heat is removed from the zones before they mix to reduce overall peak-flame temperature, thus reducing NOx, formation.
Prior oscillating combustion technology relied on the oscillating valve, which was used to introduce the desired fuel-rich and fuel-lean oscillations in the flow. In the work by Shamp et al, above, a valve such as that described in U.S. Pat. No. 5,222,713, and available from CeramPhysics, Inc., was used.
The work of Shamp et al was conducted with the objective of reducing NOx emissions. However, the valve controller used in Shamp""s work failed to provide a mechanism to provide a desired burner firing configuration, nor did it provide any mechanism for controlling or varying valve oscillating parameters such as oscillating frequency, amplitude, and duty cycle for individual burners.
There are few known combustion techniques or combustion modifications which can improve fuel efficiency in oxy-combustion furnaces. Most oxy-fuel combustion processes use standard nozzle mix oxy-fuel burners, which include separate passages for both fuel and oxygen. The fuel and oxidant generally mix at their respective nozzle ends to create mixing and a flame when ignited.
The flame is created using steady diffusion of fuel and oxygen all along the flame length. Most diffusion processes are not 100% efficient in mixing, and therefore excess oxygen (i.e., more than the theoretically correct amount) is needed. It has therefore become typical practice to supply about 5% to about 10% extra oxygen to the burner. Failure to supply excess oxygen in these amounts has in the past resulted in incomplete overall combustion and the production of undesirable CO and/or unburned hydrocarbons (HC).
It has been proposed to use flat flame oxy-burners, see U.S. Pat. Nos. 5,545,031 and 5,575,637, that can provide a larger flame surface area and thus improved radiation to the load. However, these flat-flame burners attained improvements in fuel efficiency over traditional cylindrical flame burners, e.g. U.S. Pat. No. 5,199,866 and U.S. Pat. No. 5,620,316, only on the order of 3% to 4%. Because the accuracy of fuel flow metering instruments is not great, these flat-flame fuel efficiency improvements have been marginal at best.
Other proposed techniques to reduce oxygen consumption in an oxy-fuel furnace include heat recovery using waste gases to preheat oxygen/fuel and/or the raw material. These techniques require a capital investment, and the cost of the equipment for conducting heat recovery must provide a reasonable payback in terms of energy savings in order for these techniques to be effective, a payback that may prove difficult to realize.
Yet another proposed method includes using oxy-air-fuel combustion. This involves a simple enriched-air combustion method where industrial oxygen consumption is less due to the utilization of ambient or preheated air (containing nitrogen). However, the fuel consumption and NOx emissions are much higher compared to 100% oxygen-fuel combustion depending upon the level of oxygen enrichment.
According to a first exemplary embodiment, a system useful for processing a material using heat comprises a furnace having an interior space, a sidewall, and an exhaust gas outlet, at least one burner positioned to direct a flame into the furnace interior space when fuel and an oxidant are supplied to the at least one burner, at least one valve in fluid communication with the at least one burner, the at least one valve having an open condition and a closed condition, the at least one valve passing fuel to the at least one burner when the at least one valve is in the open condition and when fluidly connected to a source of fuel, the at least one valve being movable between the open and closed conditions, and an automated logic control device operatively connected to the at least one valve to open and close the at least one valve, the automated logic control device including logic configured to control at least one of a frequency at which the at least one valve is opened and closed, the duty cycle of the at least one valve, and the flow rate amplitude of flow through the at least one valve.
According to a second exemplary embodiment, a process of controlling at least one burner in a furnace, the at least one burner receiving fuel from at least one valve and receiving oxidant from a source of oxidant comprises the steps of determining a valve duty cycle, a valve oscillation frequency, and a fuel flow amplitude through the valve, and oscillating the flow of fuel through the valve to the burner according to the duty cycle, oscillation frequency, and fuel flow amplitude.
Still other objects, features, and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings.